The present disclosure relates to processes and apparatus for in-situ measuring of absolute thickness and removal rate, and processes for predicting process endpoint or combinations thereof.
The fabrication of integrated circuits consists primarily of the deposition and removal of thin films. Various thin films deposited and removed during the course of fabrication include, among others, photoresists, dielectric materials, conductive materials, diffusion barriers, anti-reflection coatings, passivation layers, and the like. Characterizing thickness related measurements such as thickness, removal rate, and endpoint for these thin films is important for numerous well-known reasons. Characterization must often be measured before, during, and after thin film fabrication.
Optical methods are generally employed for measuring various properties of thin films. For example, optical emission spectroscopy (OES) has previously been used to determine the end point of an etching process by providing information about the etching gas and the by-product of the etching gas. The technique relies on the change in the emission intensity of characteristic optical radiation from the dielectric by-product in the plasma. Excited atoms or molecules emit light when electrons relax from a higher energy state to a lower energy state. Atoms and molecules of different chemical compounds emit a series of unique spectral lines. The emission intensity for each chemical compound within the plasma depends on the relative concentration of the chemical compound in the plasma. A typical optical emission spectroscopy apparatus operates by measuring the emission intensities of the reactive etching gas and the by-product of the etching gas and the dielectric. The emission of the by-product decreases and finally stops when an endpoint is reached. The optical emission spectroscopy apparatus senses the declining emission intensity of the by-product to determine this endpoint.
For example, using a barrel type plasma reactor, it has been reported that the light emission intensities at 283 nm from CO* and OH* emission, light emission intensities at 297.7 nm, 483.5 nm, and 519.8 nm from CO*, and light emission intensity at 308.9 nm can be used to determine the amount of photoresist removed, which can be used for determining endpoint by integration of these intensities. In a similar manner, the optical emission signal at 519.6 nm in a parallel plate reactor has been reported as suitable for detecting endpoint. In a downstream plasma reactor, an investigation of abrupt changes in intensities at 309 nm from OH and 431.5 nm from CH* have been used for determining endpoint. However, this technique generally requires an initial thickness measurement from some other method and more importantly, does not produce very accurate endpoint detection.
Other optical systems measure emission intensity at the 431 nm line and combine the data with a statistical method for determining endpoint, but in practice, the results correspond to about 60 to about 70 percent of the actual photoresist removed. To acquire more accurate endpoint detection, the optics have been set to point toward the substrate using optical emission interferometry (OEI) and the 309 nm and 431 nm lines have been used at the same time to determine endpoint. However, this type of system cannot be used for predicting endpoint for the photoresist stripping process in advance.
In downstream plasma reactors with lamp-based heaters for ashing photoresist, the noise from the lamp-based heaters and the speed of the ashing process relative to reactive ion etching (RIE) processes make it difficult to obtain accurate measurements. The radiant wafer heating lamps used to promote the ashing process interferes with the light emitted at the wafer surface. The single wavelength OEI processes simply do not behave well with non-constant light sources in measuring a large dynamic range of thickness.
The geometry of reactor chambers can also prevent the use of OEI, which heavily rely on an incident angle normal to the plane of the substrate. Attempts to use current OEI methods at incident angles other than 0° can result in measurement errors because the light generated at angles can be very low and subject to more noise. In addition, a double frequency component is encountered at high angles of incidence and the low AC/DC ratio associated with the shallow angle further exacerbates signal quality. To deal with this noise, laser light has been employed as the light source, however, alignment issues result as well as localized heating which may give an increased removal rate at the measuring location. Moreover, as previously mentioned, a single wavelength method requires the initial film thickness to be made by some other method since it only provides a rate of thickness change.
Moreover, under the high temperatures that occur during the ashing process, the photoresist can expand and shrink. The expansion and shrinkage of the photoresist layer has never before been reported and is likely to be a source of error for current measurement systems.
It should be noted that the prior art processes fail to accurately provide in-situ thickness, removal rate, and endpoint. There accordingly remains a need in the art for an in-situ non-destructive thickness measurement apparatus and process for determining real-time thickness, ashing rate, and endpoint.